1. Technical Field
The present invention relates to an image display device.
2. Related Art
Head-mounted displays (HMDs) are known as display devices that directly radiate lasers to retinas of pupils and cause users to view images.
Head-mounted displays generally include light-emitting devices that emit light and scanning units that change light paths so that the emitted light scans retinas of users. Such a head-mounted display enables a user to simultaneously view both of, for example, an outside scenery and an image depicted by the scanning unit.
For example, JP-A-2014-78022 discloses an image display device including a light source, a scanning unit that scans parallel light emitted from the light source, and an optical device that relays and emits the parallel light scanned by the scanning unit toward eyes. Of these units, the optical device is disclosed which includes a light-guiding plate configured to propagate incident light by total reflection therein and then emits the light, a first diffraction grating member that diffracts the light so that the light incident on the light-guiding plate is totally reflected, and a second diffraction grating member that diffracts the light so that the light totally propagated by the total reflection can be emitted from the light-guiding plate.
In the image display device disclosed in JP-A-2014-78022, the light scanned by the scanning unit is configured to be incident on the first diffraction grating member. Since the light incident on the first diffraction grating member is scanned over a two-dimensional scanning range with a constant area, it is necessary for the first diffraction grating member to have a sufficient size so that the light can be received. As a result, the size of the image display device disclosed in JP-A-2014-78022 may be necessarily increased.